Electric heaters for heat-treatment of a workpiece, particularly for wafer processing, are generally known in the art. A typical electric heater for wafer processing generally has a dielectric substrate with a heating surface for heating and supporting a wafer and a resistive heating element embedded inside the substrate. The substrate is generally made of ceramic materials due to, among other things, their high thermal conductivity, which facilitates the heat transfer from the resistive heating element to the heating surface and hence the wafer supported thereon. During processing, thin film deposition is to be performed on the wafer when the wafer is heated by the heater. Since a uniform heating/temperature profile on the heating surface is critical to the quality of the deposited film on the wafer, various attempts have been made to achieve a more uniform heating/temperature profile across the heating surface of the substrate.
It has been found that a high degree of uniform heating can be achieved by, among other things, (1) providing an extreme degree of flatness of the supporting surface of the substrate, (2) selecting a thermally conductive substrate with high thermal conductivity, (3) providing a thermally conductive substrate that has rigidity, stiffness, and thermal properties to achieve the desired flatness and thermal conductivity.
Despite these attempts, it is still difficult to realize a heater with an improved uniform heating/temperature profile. The processes for manufacturing a heater, particularly a ceramic heater, are complex. Factors affecting the performance and quality of a completed ceramic heater during manufacturing include, among other things, the inhomogeneous material properties of the substrate and the non-uniform temperature distribution in the manufacturing environment to which the substrate is exposed. These factors complicate the design and manufacturing aspects of a ceramic heater.
Another problem that complicates the design of a ceramic heater is the occurrence of heat loss during operation. Heat losses generally occur at the periphery and the center of the substrate, and it is difficult to predict the amount of heat losses at a specific operating temperature. If the actual heat losses during operation is greater than a maximum design limit, practically nothing can be done to remedy the situation to achieve a more uniform temperature profile once the heater is completed. As a result, the completed ceramic heater with an unsatisfactory heating/temperature profile is often scrapped, thereby increasing manufacturing costs.
It has been proposed that dividing the substrate into multiple heating zones and independently controlling the temperatures of the multiple heating zones may somehow compensate for the heat loss during operation to maintain a more uniform temperature across the heating surface of the substrate, as proposed in U.S. Pat. No. 6,423,949 (“the '949 patent”) to Chen et al. However, the proposed heater cannot deal with the situation where any one of the temperature profiles of the individual heating elements deviates significantly from the design requirements. As with a heater with a single heating zone, the heating elements of the '949 patent are embedded inside the substrate and cannot be removed without destroying the heater. Practically, nothing can be done to achieve a more uniform temperature once a heater is completed if any one of the temperature profiles of the multiple heating zones deviates significantly from the design requirements. Moreover, it is difficult to predict the exact locations and areas where hot spots or heat losses occur as well as their degree. As a result, a heater with multiple heating zones as disclosed in the '949 patent does not make the design and manufacturing of a heater less complex than a heater with a single heating zone.